The 1999 Lancer MX only has a 1.6. It's
claimed to have 124 - 125 hp (the DOHC headed version shared with the GSR), but since it's an AT, it's just... okay.
The 2000 STA is a really sluggish car, poorly geared AT, powerful, engine-sucking airconditioning (

), it's a much more cozy and comfortable interior, though, and a softer ride.
The MX leather, though, if it's anything like GSR leather, is a notch hardier and more classy feeling, but the Exalta really beats it in terms of kit. In terms of performance and driving, the MX wins (though it's not as good as the Civic).
Both cars are pretty old by now, and both have their own issues (IACV and airconditioner blower on the Lancer, aircon compressor and MAF on the Sentra), and as both are AT, you have to test drive and evaluate both cars very carefully before deciding which one to buy. Bring a mechanic you know to evaluate each.
Abused manual = clutch job... $$$. Neglected automatic = AT overhaul... $$$$$$$$.
